The present invention relates to a semiconductor laser device suitably used as a light source for optical communication and to an optical communication system using the same. The present invention also relates to technology for crystal-growing a compound semiconductor as a material for an active layer included in the semiconductor laser device and the optical communication system using the same.
A conventional semiconductor laser device as a light source for optical communication employs an InP substrate and InGaAsP mixed crystals as a material for the active layer thereof. This is because the InGaAsP mixed crystals have band gap energy on the bands of 1.3 .mu.m and 1.55 .mu.m, which are low transmission loss bands of an optical fiber.
A conventional semiconductor laser device for optical communication is illustrated in FIG. 10.
The semiconductor laser device shown in FIG. 10 includes: an n-type InP substrate 101; and a mesa-shaped multi-layer structure formed on the substrate 101. The mesa-shaped multi-layer structure includes: an n-type InGaAsP light confinement layer 102; an InGaAsP active layer 103; and a p-type InP cladding layer 104. A p-type InP current blocking layer 105 and an n-type InP current blocking layer 106 are buried in the regions interposing the mesa-shaped multi-layer structure therebetween. A p-type InP buried layer 107 and a p-type InGaAsP contact layer 108 are formed so as to cover these current blocking layers and the mesa-shaped multi-layer structure. An insulating film 109 having stripe-shaped openings is deposited over the p-type InGaAsP contact layer 108. An An/Zn electrode 110 and a Ti/Au electrode 111 are formed thereon. An Au/Sn electrode 112 is formed on the reverse surface of the substrate 101.
The InGaAsP/InP semiconductor laser device shown in FIG. 10 has a problem that the threshold current and the light emission efficiency thereof are variable to a large degree with respect to the variation in temperatures. Thus, various measures to keep the temperature of the semiconductor laser device constant, e.g., using a Peltier device, have been taken. However, the price of a laser module is raised partly because of such measures.
A very small band offset .DELTA.c on the conduction band is presumably one of the reasons why the characteristics of an InGaAsP/InP semiconductor laser device are variable to a large extent with respect to the variation in temperatures. This phenomenon will be described with reference to FIGS. 11A through 11C.
FIGS. 11A and 11B illustrate cases where an active layer has a quantum well structure including barrier layers and a well layer sandwiched therebetween. If .DELTA.Ec between the barrier layers and the well layer is as small as about 100 meV and if the temperature is low, then a sufficiently large number of electrons are confined within the well layer functioning as a light-emitting region as shown in FIG. 11A. However, if the temperature rises, then the electrons are likely to overflow from the well layer owing to the thermal energy applied and cease to contribute to the emission of light. Thus, the threshold current thereof increases and the slope efficiency declines shown in FIG. 11C.
As described above, .DELTA.Ec of the InGaAsP/InP semiconductor laser device is about 100 meV, which is much smaller than that of an AlGaAs/GaAs semiconductor laser device in the range from about 200 to about 300 meV.
In view of the above-described problems, the present invention was made in order to accomplish the objects of (1) providing a semiconductor laser device having low threshold current and exhibiting high slope efficiency over a wide temperature range and an optical communication system using the same, and (2) providing a method for producing an InN.sub.x As.sub.y P.sub.1-x-y (where 0&lt;x&lt;1 and 0.ltoreq.y&lt;1) mixed crystal having excellent crystallinity suitable for the active layer of the semiconductor laser device.